The present invention relates to a back draft carburetor of the type in which a vacuum source in the throat of the carburetor is communicated to the air space above the fuel in the float bowl of the carburetor and more particularly to valve control that alternately opens and closed the passageway connecting the vacuum source to the float bowl.
Back draft carburetors of the type shown in U.S. Pat. No. 4,040,399 apply a vacuum created by a venturi effect of the throttle valve within the throat of the carburetor to the air space above the fuel in the float bowl. This application of a vacuum to the air space above the fuel in the carburetor bowl enhances the fuel economy of the engine. It has been found that with systems of this type, that the engine will go lean to the point of not running at low engine RPM's and large throttle openings. The problem has been overcome by calibrating a progression system so that the engine would run rich. However, when the RPM's would increase with the same large throttle opening, the engine would be richer than it need be, thus, fuel economy benefits were lessened. This situation was basically caused by the fact that once the back draft hole in the throttle bore is uncovered, systems such as that shown in the U.S. Pat. No. 4,040,399 patent were always operable, thus, causing the engine to run leaner than it would if the back draft system was disconnected.
The object of the present invention is to make the back draft system inoperable at low engine RPM's and large throttle openings so that the vacuum in the throat of the carburetor will not be communicated to the air space above the fuel in the float bowl. This will prevent the engine from running lean and stopping and yet permit back draft operation at higher engine RPM's so that the fuel economy benefits are retained.